A liquid crystal display belongs to a non-self emitting display, and thus light emitting device is needed to act as a light source. Such a light emitting device is generally called a backlight module. Backlight modules are commonly divided into two types: a direct-light type and an edge-light type. The conventional backlight module uses a lamp tube such as a cold cathode florescent lamp (CCFL) as a light source. However, the CCFL fails to regenerate the real colors of objects due to the low color rendering index (CRI).
For better color rendering, a light-emitting diode (LED) is deemed a better solution for the light source of backlight module in the coming market. LEDs offer benefits such as small size, low power consumption, fast response time, long operating time, and durability, etc. A color filter is usually used in the LCD for separating the three primary colors, i.e. red, blue and green from the white light. Mixing the three primary colors in different percentages may create various desired colors.
There are several methods of forming white light by LEDs. (1) A blue LED used with a yellow phosphor, commonly yttrium-aluminum-garnet (YAG) phosphor, is one of the most popular measures forming white light. However, this type of white light is formed by mixing blue light with yellow light, and the spectrum thereof is mainly shown at two wavelengths of 460 nm and 550 nm, i.e. this type of while light lacks of red and green lights, and thus a LCD adopting this type of white light fails to show real color of object. (2) A blue LED is used to excite the red and green phosphors for generating red and green lights. The red light, the green light, and the blue are mixed to form white light. However, there is serious crosstalk among the red, blue and green colors generated by this method, i.e. the bandwidths of the red, blue and green colors are overlapped. (3) A ultraviolet. LED is used to excite three or more phosphors for generating three colors of red, blue and green. This method also causes serious crosstalk. (4) Three separate red, blue and green LEDs are used to generate white light. The white light made by this method can achieve NTSC 105% or more, which is 1.5 times higher than the conventional CCFL. However, due to the different illumination efficiencies of different colors LEDs, different numbers of the red, blue and green LEDs are required for practical application. Generally speaking, the efficiency of green LED is poorer, and thus more green LEDs are needed to balance with the light amount generated from other colored LEDs. However, the more LEDs the higher cost rises, and more space needs for accommodating the LEDs.